It is well known that some power distribution systems employ an earth ground. For example, one wire of the standard household 110 volt system is directly connected to an earth ground. However, in specialized applications, a 110 volt system can be provided that is entirely isolated from ground. The same situation is found in d.c. systems wherein some employ a grounded system and some are isolated from ground. For example, in a standard automobile one pole of the 12 volt battery is directly connected to the chassis of the automobile and referred to as ground. DC power supplies which are intended to be isolated from either a chassis and/or earth ground may cause equipment malfunctions if one pole or the other is inadvertently or unintentionally grounded. Accordingly, many techniques and devices have been organized for detecting and responding to an unwanted ground on either terminal of a d.c. power supply. For example, U.S. Pat. No. 4,002,968, issued Jan. 11, 1977, to Ian Reid and assigned to the same assignee as the present invention discloses a ground fault detector for a 2-wire power supply. U.S. Pat. No. 3,801,898, issued Apr. 2, 1974, to John A. Young illustrates another circuit for detecting a resistive path to ground on either terminal of a d.c. power supply. The application of Mande et al., Ser. No. 902,030, filed May 2, 1978 and issued as U.S. Pat. No. 4,198,625 on Apr. 15, 1980, and assigned to the same assignee as the present invention, discloses a detection scheme for responding to differences in potential levels. A false ground on either leg of a d.c. power supply which should be isolated from ground may derogate the system, causing malfunctions and improper operation.
Prior art systems have been designed which are reliable and effective but which sometimes require numerous and/or expensive components and/or require adjustment or balance. This has made such systems expensive to manufacture, expensive to maintain, difficult to adjust, and/or inconvenient to repair.